The present invention relates generally to a method of detecting underground objects and more particularly to a method using a controlled source audio magnetotelluric signal to detect the depth and shape of a subsurface, electrically conductive, object.
The recovery of heavy oil often necessitates the use of injection wells where steam is forced through the subsurface ground to push heavy oil to production wells where it may be recovered. If the well operator knows where his steam is progressing underground, he can control its distribution by known techniques in order to direct the steam toward his, and away from his neighbors', production wells. One problem is that he often does not know where his steam is progressing underground, with the result that he may be aiding the production of his neighbors' wells. At present, the operator must either drill monitor wells or use geophysical electromagnetic techniques to determine the steam distribution. The monitor wells only give spot information although they quite accurately establish the depth of the steam. The geophysical electromagnetic techniques, e.g., controlled source audio magnetotelluric (CSAMT), can provide on a real time basis an indication of the extent of a steam front, but they give a poor indication of depth.
Electromagnetic sounding techniques have been used for geophysical exploration since the 1930's. These techniques involve the measurement of one or more components of the electric and magnetic fields present in the ground. From these measurements, the apparent resistivity of the ground is calculated and the subsurface nature of the ground inferred from these calculations.
The first magnetotelluric electromagnetic sounding technique used naturally existing ground currents generated by worldwide lightning activity as the current source. Measurements of electric and magnetic field components were made in a broad range of frequencies from about 0.0001 Hz. to 100 Hz. Since lightning did not often provide sufficient signal strength for this method, in the early 1970s the technique of using a fixed electric dipole as a controlled signal source was introduced. A description of the history of this field and the present state of technology is found in "Controlled Source AMT", Zonge Engineering and Research Organization, Inc., Tucson, Ariz., 1982, which is herein incorporated by reference.
This invention utilizes existing CSAMT equipments to provide both the location and depth of underground conductive objects.